SCIENCE FILE

Tracking the Snowpack

The snow that falls in the state's mountains, particularly in the Sierra Nevada range, is an important source of water for nearly everyone who lives in California.

Agricultural, recreational and urban water users, government agencies and utility companies all rely on California's snowpack and the resulting spring runoff for their water.

Keeping track of the snowpack, as well as the water supply in reservoirs, is a full-time job for the California Department of Water Resources, which coordinates the California Cooperative Snow Surveys program. More than 50 state, national and private agencies participate in the snow survey, which collects data from more than 300 snow-course sites.

California is the only state that conducts its own survey. Every winter and spring, snow surveyors fan out to often remote areas to measure the snowpack. Crucial water agreements are based on the results of the annual April 1 survey.

This year's recently completed survey shows that the snowpack is about 110% of normal statewide.

In a classic La Nina weather pattern, the northern part of the state has received nearly as much snow as last year's El Nino-generated storms delivered, while the southern part has remained quite dry, according to state water officials.

Mirroring that pattern, snowpack measurements range from 150% of normal in the north coast area to about 60% in the southern Sierra. The eastern Sierra, where Los Angeles gets most of its water, is at about 75% of normal.

Water experts say they are confident that reservoirs will be full and water supplies will be plentiful this year.

CALIFORNIA'S SNOWFALL WATERSHEDS California's Snow-Fed River Basins

Snow that falls during winter months melts and runs off from April through July. Mountains thousands of feet high in the northern and central part of the state usually provide many feet of snow each winter, which travels down slopes and into rivers, streams and aqueducts.

HOW SNOW SURVEYORS WORK

Each of the state's snow courses is visited several times each winter by snow surveyors, who ski, snowmobile or helicopter to remote sites. Data on snowpack water content, precipitation and runoff are collected and analyzed, enabling state water officials to forecast snowmelt runoff statewide. The forecasts are published monthly starting in February, with a final report on April 1.

1. To ensure consistency from one survey to the next, surveyors record the distance between markers and the spot each sample is taken from. Most courses consist of 10 sample points.

2. A jagged-cut tube is driven into the snowpack, measuring depth and retrieving a sample to be weighed.

3. When dirt appears at the end of the sample, indicating ground level has been reached, surveyors determine snow depth from gradations on the tube.

4. Depth alone is an inaccurate measurement of the amount of water in the snowpack. Water content is determined by weighing the tube containing the snow sample and subtracting the weight of the tube from the total. The weight of the snow is an indicator of how much water it contains. An ounce of snow is equal to about one inch of water.

APRIL 1 STATEWIDE CONDITIONS

1999

Reservoir Storage: 115%

Snowpack: 115%

Precipitation: 95%

WHERE OUR WATER COMES FROM

In years of scarce rainfall, Los Angeles purchases water from the Metropolitan Water District, which wholesales Northern California water and also brings its own water from the Colorado River. Serves 3.6 million people in the city of L.A. or 650,000 water customers.

1996-97 (July 1-June 30)

Los Angeles Aqueduct: 66%

Ground water: 17%

Metropolitan Water District: 17%

*

1997-98 (July 1-June 30)

Los Angeles Aqueduct: 70%

Ground water: 18%

Metropolitan Water District: 12%

*

Historic averages:

Los Angeles Aqueduct: 65%

Ground water: 15%

Metropolitan Water District: 20%

THIS SEASON'S SNOWPACK

APRIL 1, 1999 Region: Snow Water Content, Percent of normal

Statewide: 110%

North Coast: 150%

North Sierra: 130%

Central Sierra: 100%

Southern Sierra: 60%

Eastern Sierra, North: 110%

Eastern Sierra, South: 75%

More detailed information can be found at the California Snow Page at http://cdec.water.ca.gov/snow/